The bandwidth shortage increasingly experienced by mobile carriers has motivated the exploration of the underutilized Millimeter Wave (mmWave) frequency spectrum around 30 G and 300 GHz for the next generation broadband cellular communication networks. The available spectrum of mmWave band is hundreds of times greater than the conventional cellular system. The mmWave wireless network uses directional communications with narrow beams and can support multi-gigabit data rate. The underutilized bandwidth of the mmWave spectrum has very small wavelengths, which enables large number of miniaturized antennas to be placed in a small area. Such miniaturized antenna system can produce high beamforming gains through electrically steerable arrays generating directional transmissions. With recent advances in mmWave semiconductor circuitry, mmWave wireless system has become a promising solution for real implementation. However, the heavy reliance on directional transmissions and the vulnerability of the propagation environment present particular challenges for the mmWave network with beamforming.
In principle, beam training mechanism, which includes both initial beam alignment and subsequent beam tracking, ensures that base station (BS) beam and user equipment (UE) beam are aligned for data communication. To ensure beam alignment, beam-tracking operation should be adapted in response to channel changes. However, in mmWave systems, transmission path lifetime is expected one order of magnitude shorter than traditional cellular bands due to wavelength difference. Combined with dedicated beam with small spatial coverage, the number of effective transmission paths for a dedicated beam could be rather limited, thus more vulnerable to UE movements and environmental changes.
Beam failure recovery mechanism is designed to handle the rare case beam tracking issue, e.g., when feedback rate for beam management may not be frequent enough. Beam recovery mechanism comprises triggering condition evaluation including beam failure detection and candidate beam identification, beam failure recovery request (BFRQ) transmission, and network response monitoring. Details of the beam failure recovery procedures need to be carefully designed to shorten the recovery delay while ensure the robustness. Specifically, details on BFRQ transmission and resource, and details on where and how to deliver network response need to be carefully designed.
The beam pair link (BPL) used for network-UE communication is controlled by the network. After UE recovers from beam failure, radio link failure, or handover procured, but before the network can provide regular beam indication signaling, a default BPL is required for NW-UE communication. The default BPL includes TX beam and an RX beam, which requires common understanding at both cell and UE to construct a valid default BPL. Mechanism is needed to acquire the default BPL.